Ball and socket bearing for artificial joint

ABSTRACT

A ball and socket joint for implanting in the body is provided wherein the socket portion of the joint can have various orientations with respect to the patient&#39;s anatomy, and the orientation used for a particular patient can be selected and/or changed in situ, that is, during or after implantation of the joint. In addition, the configuration of the joint, e.g., constrained versus semi-constrained, as well as the materials making up the socket portion of the joint, e.g., plastic versus metal, can be selected and/or changed in situ.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of copending application Ser. No.473,431, filed Mar. 8, 1983 no U.S. Pat. No. 4,642,123.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

This invention relates to artificial joints and in particular toartificial joints of the ball and socket type.

2. Description Of The Prior Art

As is well known in the art, artificial hip and shoulder jointsconventionally employ ball and socket articulations. The socket isembedded in one bony structure, for example, the pelvis for a hipreconstruction. The ball is attached to an arm composed of a neck and astem or shaft, the stem or shaft being embedded in another bonystructure, for example, the femur for a hip reconstruction.

A number of methods are known for retaining the ball in the socket. Inthe most common method, referred to herein as the "semi-constrained"construction, the patient's own anatomy, i.e., his muscles, tendons andligaments, are used to retain the ball within the socket. For thisconstruction, a hemispherical socket typically is used which allows theball and its attached arm the maximum amount of movement without contactof the arm with the edge of the socket. The surgeon, when installingsuch a semi-constrained joint, aligns the ball and socket as closely aspossible with the patient's natural anatomy so that the patient'smovements do not tend to dislocate the ball from the joint. As a generalproposition, such precise alignment is easiest the first time anartificial joint is placed in a patient. Subsequent reconstructions aremuch more difficult to align because of deterioration of anatomicallandmarks as a result of the first operation, the healing process afterthe operation and changes in the anatomy caused by the presence of theartificial joint.

In order to increase the inherent stability against dislocation of suchsemi-constrained constructions, it has become conventional to add acylindrical portion to the hemispherical socket to make it deeper.Although the ball is not physically constrained by the socket by thisadjustment, the ball does have further to travel than if just ahemisphere had been used and thus some reduction in the propensitytowards dislocation is achieved. Ball and socket joints of this typegenerally provide an arc or range of motion of approximately 115° when a28 mm diameter sphere is used and the socket is made a few millimetersdeeper than a hemisphere. Larger ranges of motion can be obtained bykeeping the size of the arm attached to the ball constant and increasingthe diameter of the ball. In this way, the angular extent of the armrelative to the ball becomes smaller. In the limit, if the ball could bemade progressively larger and larger, a range of motion of 180° could beachieved. In practice, however, the largest sphere in common use inartificial joints, and in particular artificial hip joints, has adiameter of 32 mm and provides a range of motion of approximately 120°.It should be noted however, that such larger sphere sizes are notuniversally favored because frictional torque increases with diameter.

A recent study by the Mayo clinic, which appeared in December, 1982edition of The Journal of Bone and Joint Surgery, reported a dislocationfrequency of 3.2% for 10,500 hip joint implant procedures using thesemi-constrained construction. Such dislocations essentially make thepatient immobile and can necessitate a second operation. As discussedabove, the critical alignment required for the semi-constrainedconstruction is even more difficult to achieve when a secondimplantation is performed. Accordingly, even higher dislocationfrequencies are encountered for second and subsequent implantations.

An alternative to the semi-constrained construction is the constructionwherein the ball is physically constrained within the socket. In thisconstruction, a spherically-shaped bearing surrounds the ball and servesas the socket. The bearing is attached to a fixation element which isembedded in, for example, the patient's pelvic bone. The bearingencompasses more than one-half of the ball and thus constrains the balland its attached arm from dislocation.

The bearing is typically made from plastic, such as ultra-high molecularweight polyethylene (UHMWPE), or metal. For plastic bearings, the balland bearing are usually assembled by forcing the bearing over the ball.The more of the ball which is encompassed by the bearing, the greaterthe required assembly force, and the greater the constraining force toprevent postoperative dislocation of the joint. In addition, the morethat the bearing encompasses the ball, the smaller the range of motionfor the ball prior to contact of the bearing with the arm attached tothe ball.

An example of a constrained artificial joint employing a plastic bearingis shown in Noiles, U.S. Pat. No. 3,996,625. As can be seen in FIG. 1 ofthis patent, a plastic bearing 17 fitted with a metal reinforcing band(un-numbered) extends beyond the diameter of ball 24 so as to physicallyconstrain the ball within the bearing. The bearing itself is attached tofixation element 12. The metal reinforcing band is assembled over thelip of the opening of bearing 17 after that bearing has been forced oversphere 24. The reinforcing band increases the force required todislocate the joint. In practice, the design shown in FIG. 1 of U.S.Pat. No. 3,966,625 has been found to provide a range of motion ofapproximately 85° when a sphere diameter of 28 mm is used and to resistdirect dislocating forces of several hundred pounds.

For constrained constructions such as that shown in U.S. Pat. No.3,996,625, it has been found in use that a dislocating force is createdwhen the neck of the arm attached to the ball impinges on the rim of thebearing. Because of the leverage associated with the arm and the longbone of the patient to which it is attached, e.g., the patient's femur,the dislocating force produced when the neck contacts the rim of thebearing can be considerable. For example, a force on the order of 25pounds applied to a patient's leg can produce a dislocating force ofover several hundred pounds because of the leverages involved. This typeof dislocation force can be avoided by geometrically aligning theartificial joint with the patient's anatomy so that the neck does notcome in contact with the rim of the bearing during normal motion of thepatient's limb. That is, the leverage based dislocation forces can beavoided in the same way as dislocations are avoided in theseim-constrained construction, i.e., through precise alignment of theartificial joint with the natural anatomy of the patient. Unfortunately,as is apparent from the geometry of the situation, the more the socketbearing encompasses the ball, the greater the restraining force on theball, but at the same time the less the range of motion prior to theneck impinging upon the edge of the bearing to create undesiredleverage. In practice, artificial hips having the construction shown inU.S. Pat. No. 3,996,625 have been found to suffer dislocation due to theleverage effect in fewer than 0.5% of the implantations performed. Thisis significantly better than the 3.5% dislocation frequency reported inthe Mayo clinic study discussed above, but an even lower dislocationfrequency is obviously desirable.

A constrained construction using a metal socket bearing is shown inNoiles, U.S. Pat. Re. 28,895 (reissue of U.S. Pat. No. 3,848,272). Thisconstruction provides approximately a 90° range of motion when thesphere diameter is 28 mm. In a practical sense, the metal bearing can besaid to be non-dislocatable. The force required to extract the metalsphere from the enclosing metal socket bearing is more than severalthousand pounds. Accordingly, in use, rather than the metal balldislocating from the metal socket bearing, any overly severe dislocatingleverage will cause the fixation element to be disrupted from the bonein which it has been embedded.

As a general proposition, metal balls in metal socket bearings are usedin only a minority of joint reconstructions because the medicalprofession is not in agreement that a metal sphere in a metal bearing isas biologically acceptable as a metal sphere in a UHMWPE plasticbearing, even though clinical use over 15 years has failed to show themetal to metal joint to be inferior to a metal to plastic joint.

A third type of artificial ball and socket joint, referred to as anendoprosthesis, eliminates the fixation element associated with thesocket and simply uses a ball surrounded by a plastic socket bearing ina spherical metal head, which head is placed in the patient's naturalsocket but not secured to bone. For this construction, the ball canrotate within the bearing up to the rim of the bearing (the bearing isgreater than a hemisphere so as to be retained on the ball), and thenthe bearing and its attached head rotates in the patient's socket. Aswith the semi-constrained construction, anatomical alignment is used toavoid dislocations, in this case between the metal head and the naturalsocket.

In view of the foregoing, it is apparent that in semi-constrained andendoprosthesis hip joints, reconstructive geometry of the prostheticcomponents is critical in ensuring the stability of the prosthesisagainst dislocation. Moreover, in ball and socket constructions whichconstrain the elements against dislocation, the range of motion inherentin the prosthesis is reduced and thus because of the possibility ofleverage type dislocations, similar demands are placed on the surgeon toestablish the geometry of the reconstruction within rather narrowlimits.

Accordingly, an object of this invention is to provide a ball and socketjoint which provides the surgeon with increased latitude in geometricpositioning of the prosthetic components over those ball and socketjoints presently available.

A further object of the invention is to provide a ball and socket jointthe materials and configuration of at least a portion of which can beselected and/or changed in situ, that is, during or after implantationof the joint in the patient.

An additional object of the invention is to provide a ball and socketjoint including a bearing member which can be readily replaced in situwith either a bearing member of the same or of a different typedepending on the patient's post-operative history.

Another object of the invention is to provide a ball and socket jointwherein (1) the socket portion of the joint has more than oneorientation with respect to the ball portion of the joint, the preferredorientation being a function of the patient's anatomy, and (2) theorientation of the socket portion with respect to the ball portion canbe selected and/or changed in situ, that is, during or afterimplantation of the joint in the patient.

A further object of the invention is to provide a prosthetic ball andsocket joint of increased inherent range of motion which is readilyassembled and disassembled at the surgical site.

An additional object of the invention is to provide a ball and socketbearing for an artificial joint which constrains the joint fromdislocating and at the same time provides a range of motion which isgreater than that available in the constructions of the constrained typedescribed above.

SUMMARY OF THE INVENTION

To achieve these and other objects, the invention, in accordance withone of its aspects, provides a ball and socket joint for implantation ina patient's body comprising a ball portion and a socket portion,

the ball portion including:

a ball; and

first fixation means for implantation in a first bony structure, saidfixation means being connected to said ball; and

the socket portion including:

a bearing for receiving the ball; said bearing defining an orientationbetween itself and the patient's body;

second fixation means for implantation in a second bony structure; and

connecting means associated with the bearing and the second fixationmeans for connecting the bearing to the second fixation means in morethan one orientation.

In accordance with a further one of its aspects, the invention providesa ball and socket joint for implantation in a patient's body comprisinga ball portion and a socket portion,

the ball portion including:

a ball; and

first fixation means for implantation in a first bony structure, saidfixation means being connected to said ball; and

the socket portion including:

a bearing for receiving the ball, said bearing being one member of afamily of interchangeable bearings, the family including at least onemember which is made from a different material or which has a differentconfiguration or which is both made from a different material and has adifferent configuration from at least one other member of the family;

second fixation means for implantation in a second bony structure; and

connecting means associated with the bearing and the second fixationmeans for interchangeably connecting any bearing in the family to thesecond fixation means.

In accordance with an additional one of its aspects, the inventionprovides a ball and socket joint for implanting in the body whichcomprises:

a ball;

a cup with a spherical cavity, said cup to be affixed to bone; and

a bearing member surrounding a portion of the ball and rotatable withinsaid spherical cavity about only one axis, said bearing member having anasymmetric opening therein, the opening having an angular extent of lessthan 180° in at least one plane.

In accordance with another one of its aspects, the invention provides anartificial joint of the ball and socket type for implantation in thebody which comprises:

a ball;

a bearing for forming a socket to receive the ball, the bearing havingan asymmetric opening therein, the opening having an angular extent ofless than 180° in a first plane;

means for pivoting the bearing about an axis lying in a plane other thanthe first plane; and

means for affixing the means for pivoting to bone.

In accordance with another of its aspects, the invention provides asocket for a ball and socket joint for implantation in the body whichcomprises (1) a cup with a cavity, and (2) a bearing for receiving theball of the ball and socket joint, said bearing being constrained torotate within said cavity about a single axis.

In accordance with certain preferred embodiments of the invention, theasymmetric opening into the socket is less in one direction than it isat 90° to this one direction. The socket bearing is movably retainedwithin the cup about an axis which is (1) parallel to the face of thecup and (2) in the direction of the greater opening in the socketbearing. The socket bearing is retained within the cup by two stub halfround pins integral with the cup and extending part way through the wallthickness of the socket. The axes of the half round pins coincide withan axis of the spherical cup like cavity and they are also coaxial inthe direction of the greater opening in the socket bearing.

When the ball and the neck of the arm of the prosthesis move in thedirection of the lesser opening in the socket bearing, the total rangeof motion is the sum of the arc of motion which the neck can make withinthe bearing plus the arc of motion which the bearing can make within thecup. The cup can be a hemisphere or even less. Rotation of the ball islimited by impingement of the neck against the rim of the cup, oralternatively, and most preferably, by limiting the rotation of thesocket bearing so that the neck comes just up to, but not actually intocontact with, the rim of the cup. In this regard, reference is made tocopending U.S. patent application Ser. No. 553,518 to Alfred FrederickDeCarlo, Jr., filed simultaneously herewith and assigned to the assigneeof the present application. This application, the pertinent portions ofwhich are incoporated herein by reference, discloses a preferred systemfor limiting the rotation of the socket bearing to keep the neck of thearm of the prosthesis out of contact with the rim of the cup.

When the diameter of the ball is approximately the 28 mm in common use,and the socket bearing wall thickness is approximately 7 mm, the innerdiameter of the cup, and thus the outer diameter of the bearing, isapproximately 42 mm (28 mm+7 mm+7 mm). This outer diameter for thebearing is larger than the largest diameter sphere commonly used insemi-constrained artificial hip replacements, and thus the presentconstrained construction achieves a greater range of motion than thesemi-constrained construction, and at the same time, restrains the ballwithin the socket.

When the ball and the neck move in the direction of the greater openingin the socket bearing, the neck contacts the flat side of a stub halfround pin, rather than the rim of the cup, or alternatively, and mostpreferably, a web portion of the socket bearing in the region of thestub half round pins (see, for example, element 106 in FIGS. 13, 15 and21 below). To allow the neck and ball to move through the same arc inthis direction, the flat sides of the pins can be contoured. With thisfeature, the total range of motion in all quadrants, using the abovedimensions, is approximately 135°.

To summarize, in accordance with the above preferred embodiments of theinvention, when motion is in the plane of the stub pins, the totalmotion is by movement of the ball within the bearing. When motion is at90° to the plane of the pins (the "90° plane"), the total motion is thesum of the motion of the ball within the bearing and the motion of thebearing within the cup. In other planes, the motion of the ball withinthe bearing is greater than it is in the 90° plane and the motion of thebearing within the cup is less than it is in the 90° plane. In this way,the invention provides a constrained ball and socket prosthetic jointwith a total range of motion significantly greater than hithertogenerally available.

In connection with artificial hip joints, it is advantageous to orientthe cup in situ so that the axis of the stub pins is inclined accordingto the anatomical requirements of the patient as determined by thesurgeon. For example, the axis can be inclined somewhat upward in theforward direction. In this manner almost all highly repetitive loadbearing motions of the hip joint fall within the motion capability ofthe sphere within the socket bearing. Additional motion is furnished bymovement of the bearing within the cup in such activities as crossingthe legs when seated, or in significant abduction. To convenientlypermit such orientation of the stub pins, in certain embodiments of theinvention, the cup includes first and second portions, the first portionto be affixed to bone, the second portion having associated therewiththe pin members and being moveable with respect to the first portion toprovide a plurality of possible orientations for the axis of rotation ofthe bearing member within the spherical cavity.

In connection with both artificial hip joints and other types of balland socket joints, it is advantageous for the surgeon to have as wide arange of joint configurations and materials to choose from as possible.It is particularly advantageous for the surgeon to be able to refine hisselection of materials and configurations during the operativeprocedure, after he has seen the diseased joint and has a fullappreciation of the patient's medical condition and anatomy. Along thesesame lines, it is also advantageous to be able to re-operate and changematerials and/or joint configurations as a function of the patient'spost-operative history without substantially disturbing the establishedfixation of the joint to bony structures. For example, a patientoriginally fitted with a semi-constrained joint may be found to beespecially prone to dislocations so that a constrained construction,perhaps including a metal socket, would be more appropriate.

To achive these types of flexibility, in accordance with certainpreferred embodiments of the invention, a family of interchangeablesocket bearings of different configurations and/or materials is providedto the surgeon. Each of the bearings includes means for interchangeablyconnecting the bearing to a fixation element for the socket portion ofthe joint in such a way that the bond between the fixation element andthe patient's bone is not substantially disturbed by the connectingprocess. In view of this easy interchangeability, during the initialoperation, the surgeon need not choose the specific socket bearing to beused until after completing the implantation of the fixation element,and during subsequent operations, if any, he can substitute a differentbearing or replace a worn bearing without breaking the bond between thefixation element and the patient's bone.

In the description of the preferred embodiments which appears below,constructions are shown using both plastic and metal socket bearings, aswell as bearings employing a combination of metal and plasticcomponents. Also, various assembly and disassembly constructions areillustrated. It is to be understood, of course, that both the foregoinggeneral description and the following detailed description areexplanatory only and are not restrictive of the invention.

The accompanying drawings, which are incorporated in and constitute partof the specification, illustrate the preferred embodiments of theinvention, and together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an artificial joint embodying thepresent invention.

FIG. 2 is an exploded view showing the components of the joint of FIG.1.

FIG. 3 is a cross-sectional view along lines 3--3 in FIG. 1 showing theball and the socket bearing partially inserted into the cup.

FIG. 4 is a cross-sectional view along lines 3--3 of FIG. 1 showing therange of motion of the ball within the socket bearing with the socketbearing stationary.

FIG. 5 is a cross-sectional view along lines 3--3 of FIG. 1 showing therange of motion of the ball within the socket bearing when the socketbearing moves within the cup.

FIG. 6 is an exploded view of an alternate embodiment of the inventionwherein the cup includes two portions which are moveable relative toeach other.

FIG. 7 is a cross-sectional view along lines 7--7 in FIG. 6 after thejoint has been assembled.

FIG. 8 is a cross-sectional view along lines 8--8 in FIG. 6 after thejoint has been assembled.

FIG. 9 is an alternate embodiment of the embodiment shown in FIGS. 6-8wherein a two piece metal socket bearing is used.

FIG. 10 is a cross-sectional view along lines 10--10 in FIG. 9 after thejoint has been assembled.

FIG. 11 is an exploded view showing an alternative method for rotatablyretaining the socket bearing within the cup.

FIG. 12 is a cross-sectional view along lines 12--12 in FIG. 11 afterthe joint has been assembled.

FIGS. 13 and 14 show alternative socket bearings for use with thepresent invention.

FIG. 15 is an exploded view of an artificial joint similar to that shownin FIGS. 6-8 but employing a system of the type disclosed in the DeCarlopatent application referred to above to limit the range of motion of thesocket bearing within the joint.

FIG. 16 is a cross-sectional view along lines 16--16 in FIG. 15.

FIG. 17 is a cross-sectional view along lines 17--17 of FIG. 15.

FIG. 18 is a perspective view of the outer surface of the bearing memberof FIG. 15.

FIGS. 19 and 20 are schematic diagrams illustrating the relationshipsbetween the angular extents of the various components of the artificialjoint shown in FIGS. 15-17.

FIG. 21 is an exploded view of an artificial joint of thesemi-constrained type embodying the present invention and employing asystem of the type disclosed in the DeCarlo patent application referredto above to limit the range of motion of the socket bearing within thejoint.

FIG. 22 is a cross-sectional view along lines 22--22 in FIG. 21.

FIG. 23 is a cross-sectional view along lines 23--23 in FIG. 21.

FIG. 24 is an exploded view of an artificial joint of the constrainedtype constructed in accordance with the present invention and includinga metal reinforcing band to increase the amount of force required todislocate the joint. This embodiment also employs a system of the typedisclosed in the DeCarlo patent application referred to above to limitthe range of motion of the socket bearing within the joint.

FIG. 25 is a cross-sectional view along lines 25--25 in FIG. 24.

FIG. 26 is a perspective view of a plastic socket bearing constructed inaccordance with the present invention and designed to produce acompleted joint of the non-rotating, constrained type.

FIG. 27 is a front view of the bearing of FIG. 26.

FIG. 28 is a cross-sectional view along lines 28--28 in FIG. 27.

FIG. 29 is a cross-sectional view along lines 29--29 in FIG. 27.

FIG. 30 is a side view of the bearing of FIG. 26.

FIG. 31 is a side view of a plastic socket bearing constructed inaccordance with the present invention and designed to produce acompleted joint of the non-rotating, semi-constrained type.

FIG. 32 is a side view of a plastic socket bearing similar to thebearing of FIG. 31 but including a lip to help restrain dislocations ofthe completed joint.

FIG. 33 is an exploded view of an artificial joint of the non-rotating,constrained type constructed in accordance with the present inventionand employing a metal socket bearing.

FIG. 34 is a cross-sectional view along lines 34--34 in FIG. 33.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to the drawings, wherein like reference charactersdesignate like or corresponding parts throughout the several views,there is shown in FIG. 1 an assembly 20 of ball or sphere 10, socketbearing 12 and cup 14 for a prosthetic joint. The neck 16 of arm 30 isintermediate ball 10 and stem or shaft 18, which stem or shaft is fixedto, for example, the femur bone at the time of implant surgery.

FIG. 2 shows in more detail socket bearing 12 of assembly 20. Thepreferred material for bearing 12 is ultra-high molecular weightpolyethylene (UHMWPE). Inner spherical bearing surface 21 of bearing 12is concentric with outer spherical bearing surface 22. Cylindricalsurfaces 24 are coaxial with each other and with the center of sphericalsurfaces 21 and 22 and are tangent to surfaces 26. Small barb-shapedprotuberances 28 serve a detent function described below.

In the plane passing through the lines P--P in FIG. 2, socket bearing 12encompasses less than one half of ball 10. In the plane passing throughthe lines S--S, the socket bearing encompasses more than half of ball10.

Owing to the resilience and elasticity of the plastic material of socketbearing 12, socket bearing 12 can be snapped over ball 10. The amount ofinterference between the equator of the ball and socket bearing 12depends on the angular extent of the bearing's opening in the planepassing through the lines S--S in FIG. 2. the amount of interferenceshould be such as will cause an elastic deformation of socket bearing 12while the bearing is being assembled over the ball 10. To aid inassembly, socket bearing 12 can be heated to a non-destructivetemperature (for example 70°-80° C. for UHMWPE). Plastic in general, andUHMWPE in particular, has a large coefficient of thermal expansion andsuch thermal expansion due to heating significantly aids in assembly.

As shown in FIG. 2, cup 14 has a hemispherical inner surface 32 and twocoaxial stub half pin members 34 which are structurally integral withcup 14. The pins 34 are shown bevelled at 36. Recesses 38 are providedat the inner rim of the cup at locations 90° displaced from the pins 34.The exterior surface 40 of cup 14 is of any conventional contour forfixation in bone whether by use of cement, or without cement by means ofimpaction, screwing in, or by bone ingrowth into porous metal or thelike. Cup 14 is normally made of metal, and it is to be understood thatthe metal used is to be structually and biologically suitable forsurgical implantation.

A step midway in the process of assembly is schematically shown in FIG.3 where bearing 12 has been positioned against neck 16 at 42 and thebearing 12 and arm 30 have been inserted into cup 14 with neck 16contacting the rim of cup 14 at 44. Cylindrical surface 24 of bearing 12engages stub pin 34 as the entering rim 46 of bearing 12 contacts innersurface 32 of cup 14. At this time the bearing 12 is pressed firmlyenough into cup 14 to compress protuberance 28, allowing bearing 12 tobe rotated clockwise about ball 10 and pin 34 while it is in contactwith inner spherical surface 32.

When bearing 12 has been rotated sufficiently for protuberance 28adjacent rim 46 to reach recess 38, protuberance 28 expands to resistrotation in the reverse direction and thereby resist disassembly of theball and socket joint 20 unless a tool is inserted into recess 38 toagain compress protuberance 28 as rotation in the disassembly directionis started.

The assembled joint is shown in FIG. 1, where the neck 16 of arm 30 canmove through the arc from the position shown to that which issymmetrically opposite. That is, the neck of the prosthesis can move inthe plane through lines P--P of FIG. 2 from a position of contact withlower stub pin 34 to contact with upper stub pin 34. When ball 10 has adiameter of 28 mm and the outer diameter of bearing 12 is 42 mm, the arcor range of motion of neck 16 is somewhat greater than 135°, dependingon the design of the neck 16.

To achieve this same range of motion in the plane through lines S--S inFIG. 2 requires two motions. First, as shown in FIG. 4, the neck 16 andball 10 move through the angle A by the ball 10 turning inside thesocket bearing 12, at the completion of which neck 16 contacts the rimof bearing 12. Thereafter, as shown in FIG. 5, to achieve the full rangeof motion A', ball 10 and bearing 12 rotate in unison, at the completionof which neck 16 contacts cup 14.

Normally, until neck 16 reaches the rim of socket bearing 12, socketbearing 12 will remain stationary relative to cup 14. This is so becausefrictional torque is the product of friction force times the distancefrom the center of rotation. Given similar materials, finish andgeometric accuracy, so that the coefficient of friction for ball 10 andcup 14 against bearing 12 are equal, the frictional force on innersurface 21 will be the same as that on outer surface 22 when ball 10rotates within cup 14, because the load transmitted across the twobearing surfaces is the same. Since the radius to the outer surface 22is the greater, the frictional torque at the outer surface will be thegreater and thus motion will occur along surface 21 rather than surface22.

For major oscillation of ball 10 and neck 16 in the plane through linesP--P in FIG. 2, the entire excursion is due to rotation of ball 10within bearing 12. The total possible oscillation in all planes is thesame, however, the contribution made by rotation of bearing 12 increasesas the plane of oscillation moves from that including the lines P--P tothat including the lines S--S in FIG. 2.

As described above, bearing 12 constrains ball 10 from dislocation.Further, socket bearing 12 is constrained within cup 14 by cylindricalsurfaces 24 being journaled by the stub half pins 34 in all positions ofbearing 12 as bearing 12 moves to allow arm 30 to move through angle A'.In the complete assembly 20, the constraint against dislocation of ball10 by deformation of plastic bearing 12 is greater in magnitude than theforce required to assemble bearing 12 over ball 10 because, in additionto the fact that the assembly operates at the body temperature of 37°C., the bearing 12 is now itself constrained against the deflection ofdislocation by being captured within metal cup 14.

FIGS. 6, 7 and 8 show an alternate construction intended to (1)facilitate final assembly at the operative site, and (2) for hip jointreplacements, allow the axis of stub pins 34 to be inclined accordinglyto the anatomical requirements of the patient as determined by thesurgeon. Cup 14 now includes two portions--portion 64 which is affixedin the patient's bone, and retaining ring portion 74 which carries stubpins 34 and is engageable with portion 64 at a number of locations toprovide a plurality of orientations for the axis through pins 34 aboutwhich socket bearing 12 rotates. Portion 64 is shaped to accept and holdretaining ring 74 by means of bayonet spaces 68 and lugs 70. Innerspherical surface 72 is continuous with spherical surface 76 of ring 74.Ring 74 carries stub half pin members 34, has recesses 38 and bayonetlugs 78. A particularly preferred construction for portion 64, and, inparticular, for the exterior surface of this portion, is disclosed incopending U.S. patent application Ser. No. 553,519 to Douglas G. Noiles,filed simultaneously herewith and assigned to the assignee of thepresent application. The pertinent portions of this application areincorporated herein by reference.

With the embodiment shown in FIGS. 6-8, the portion 64 of cup 14 isimplanted in the patient's bone by conventional techniques, or, mostpreferably, by the techniques described in the above-referencedcopending application to Douglas G. Noiles. Ball 10 and bearing 12 areassembled into retaining ring 74 after stem 18 of arm 30 has beenimplanted in, for example, the patient's femur, the assembly procedurebeing the same as that described above with reference to FIG. 3 exceptthat protuberance 28 is compressed only once it contacts the backsurface of ring 74. The sub-assembly of ball 10, bearing 12 andretaining ring 74 is then inserted into portion 64 in any of the severalangular positions the bayonet lug fittings will permit. A fraction of aturn in either direction will engage the lugs 78 of ring 74 under lugs70 of portion 64. Alternatively, lugs 78 can be beveled at either theirright or left hand leading edges so that insertion by rotation in onlyone direction is facilitated, e.g., clockwise rotation. The engagementof bayonet lugs 78 and 70 is locked by conventional means, such as byone or more pins 80. Holes 79 and 81 for such locking pins can beprecisely made in the cooperating parts at the time of manufacture.Although only one hole 81 is shown in FIGS. 6 and 7, a hole wouldnormally be drilled at each bayonet space 68 so that ring 74 can belocked in place for any of its possible orientations.

An embodiment of the present invention similar to that shown in FIGS.6-8 but employing the system disclosed in the DeCarlo patent applicationreferred to above is shown in FIGS. 15-20. In this embodiment, sphericalsurface 72 has associated therewith pin or projection 50. Thisprojection is located at the geometric pole of the spherical cavityformed by spherical surfaces 72 and 76. As shown in the figures, pin 50has sloping sides 60.

Projection 50, in combination with aperture 13 formed in outer surface22 of bearing member 12, serves to constrain the rotation of bearing 12so as to prevent the bearing from being rotated out of the sphericalcavity once the joint is assembled and to limit the rotation of thebearing so as to keep neck 16 just out of contact with the rim of ring74, e.g., on the order of a half a millimeter above the rim. Inparticular, bearing 12 can rotate only to the point where polar pin 50and one of the end walls 52 or 54 of aperture 13 are in engagement. Asdiscussed below, this constrained condition for bearing 12 occursautomatically as the joint is assembled without any additional assemblysteps. Also, the constraining of socket bearing 12 within the joint isaccomplished irrespective of the angular orientation chosen forretaining ring 74 with respect to body portion 64 of cup 14.

Aperture 13 has a long axis parallel to side walls 56 and 58 and a shortaxis at 90° to side walls 56 and 58. The angular extent of aperture 13along its short axis is sufficient to accommodate polar pin 50. Theangular extent of aperture 13 along its long axis determines the rangeof motion of socket bearing 12. As discussed above, a particularlypreferred range motion for bearing 12 is one in which neck 16 is keptjust out of contact with ring 74. In this case, as shown in FIGS. 19-20,the angular extent (α) of aperture 13 along its long axis is determinedby: (1) the maximum angular extent (β) of socket bearing 12; (2) theminimum angular extent (γ) of cup 14; (3) the angular extent (δ) ofpolar pin 50; and the angular offset (ε) of neck 16 from cup 14. Inparticular, the angular extent of aperture 13 is given by:

    α=β+δ-γ-ε.

Similar relationships can be derived for other desired ranges of motionfor socket bearing 12.

The placement of pin 50 at the pole of the spherical cavity formed bysurfaces 72 and 76 allows retaining ring 74 to be inserted into bodyportion 64 of cup 14 in any of the possible orientations provided by themating of bayonet lugs 78 with bayonet lugs 70. That is, once socketbearing 12 is rotated about stub pins 34 until at least some portion ofaperture 13 is located over the central axis of ring 74, ring 74 can bemated with body portion 64 in any of their possible relativeorientations, because, for each of those orientations, aperture 13 willslip over projection 50. Since placing aperture 13 about pin 50 resultsin the restraining of bearing 12 in cup 14 without any further action bythe surgeon, it can be seen that assembly of the joint automaticallyproduced the desired restraining function.

A typical sequence of steps for implanting the prosthesis of theembodiment shown in FIGS. 15-20 in a patient are as follows. Stem 18 ofarm 30 is implanted by conventional techniques in, for example, thepatient's femur bone. Body portion 64 of cup 14 can also be implanted byconventional techniques, or, most preferably, by the techniquesdescribed in the above-referenced copending application to Douglas G.Noiles. Bearing 12 is assembled into ring 74 and then ball 10 is forcedinto bearing 12. Alternatively, bearing 12 can first be placed on ball10 and that combination assembled into ring 74. In either case, thesub-assembly of ball 10, bearing 12 and retaining ring 74 is theninserted into body portion 64 in any of the several angular positionsthe bayonet lug fittings will permit, with polar pin 50 sliding intoaperture 13. A fraction of a turn in either direction will engage thelugs 78 of ring 74 under lugs 70 of portion 64. Alternatively, asdiscussed above, lugs 78 can be beveled at either their right or lefthand leading edges so that insertion by rotation in only one directionis facilitated, e.g., clockwise rotation. To aid in the rotation of ring74, the ring can include apertures 122 for engagement with a spannerwrench or the like. Note that because of the polar location of pin 50,ring 74 and its attached bearing 12 can be rotated to engage lugs 78 and70 irrespective of where pin 50 is located along the length of aperture13. The engagement of bayonet lugs 78 and 70 is locked by one or morescrews 83 which pass through openings 100 and 98 in lugs 70 and 78,respectively, and then through holes 102 to engage the bone into whichcup 64 has been implanted.

For hip joints, the possibility of a number of orientations for the axisof rotation of bearing 12 is used to place that axis in an orientationin which the greater required range of motion is aligned approximatelywith axis P--P. For example, the axis of rotation can be oriented upwardin the forward direction to achieve this result. In this way, almost allof the highly repetitive lead bearing motions of the joint will occuralong or close to this axis. As discussed above, motions along or nearto the axis of rotation of bearing 12 consist primarily of ball 10moving in bearing 12, rather than bearing 12 moving in cup 14. As alsodiscussed above, the frictional torques involved further favor movementof ball 10 in bearing 12. Accordingly, by placing the axis of rotationof bearing 12 in a favorable orientation, most repetitive motion willoccur by movement of ball 10. This is an important advantage because itmeans that the joint will have low friction in that friction increaseswith the diameter of the moving member and ball 10 has a smallerdiameter than bearing 12. Put another way, by orienting the axis ofrotation of the bearing 12 in the manner described above, the joint ofthe present invention for the great majority of motions of the patient'slimb exhibits the frictional behavior of a small ball, e.g., a 28 mmball, while providing a range of motion corresponding to a large ball,e.g., a 42 mm ball.

FIGS. 9 and 10 show another embodiment employing retaining ring 74 inwhich the socket bearing comprises two metal half bearings 82. A groove86 is formed along the junction of the bearings and ends short of theedge of the bearing to form shoulders 88. Metal half bearings 82 arebrought together to encompass ball 10, and the ball, half bearings,retaining ring 74 and portion 64 of cup 14 are assembled in the samemanner as described above in connection with FIGS. 6-8.

Screws 94 having screw heads 96 are conveniently used both to lock lugs70 and 78 in place and to prevent socket bearing 12 from rotating backout of retaining ring 74. Screw heads 96 ride in groove 86 and engageshoulders 88 when socket bearing 12 has been moved through its fullrange of motion about stub pins 34. Lugs 70 and 78 have appropriateopenings 100 and 98, respectively, to receive screws 94 and allow thescrews to be engaged with, in this case, threaded screw holes 102.Although only two openings 100 and two threaded screw holes 102 areshown in FIGS. 9 and 10, such openings and threaded holes would normallybe provided at each lug 70 so that ring 74 can be locked in place forany of its possible orientations.

As shown in FIGS. 9 and 10, and most clearly in FIG. 10, screw heads 96for the present embodiment lie above the plane of the front face ofretaining ring 74. So as to provide the same range of motion of socketbearing 12 for this embodiment as for the embodiment of FIGS. 6-8, stubpins 34 also lie above this plane, so that the axis of rotation ofsocket bearing 12 is in the plane of screw heads 96. For thisarrangement, the motion of bearing 12, and thus arm 30, is limited byscrew head 96 contacting shoulder 88, rather than by neck 16 contactingretaining ring 74.

A further embodiment of the present invention is shown in FIGS. 11 and12. This embodiment employs means other than half stub pins 34 to definethe axis of rotation of socket bearing 12 within cup 14. In particular,a dovetail arrangement is used wherein male portion 90 of the dovetailis attached to socket bearing 12 and female portion 92 of the dovetailis cut into surface 32 of cup 14. Socket bearing 12 and cup 14 areassembled in a manner similiar to that shown in FIG. 3. That is, aftersocket bearing 12 has been placed over ball 10, the ball and socketbearing are moved into cup 14 until the center of ball 10 lies at thecenter of the cup's spherical cavity. Thereafter, socket bearing 12 isrotated so that male portion 90 and female portion 92 of the dovetailengage with each other. To retain socket bearing 12 within cup 14,screws 94 can be inserted into threaded holes 104 in cup 14 so as toblock the outward passage of male portion 90 of the dovetail from cup14.

FIGS. 13 and 14 show alternate socket bearings for use with the presentinvention.

In FIG. 13, cylindrical surfaces 24 do not extend completely through thewall of bearing 12, but rather stop approximately half way through toleave webs 106. So as to provide as large a range of motion of arm 30 inthe plane through lines P-P as possible (see FIG. 2), the webs extend tojust above height of stub pins 34 at the end of bevels 36. In this way,as discussed above, the motion of ball 10 in the plane through lines P-Pis limited by arm 16 contacting webs 106. The webs, although small, helprestrain ball 10 within bearing 12.

FIGS. 14 and 21-23 show embodiments of bearing 12 which do notphysically contrain ball 10. For these embodiments, inner surface 21 ofbearing 12 has a cylindrical shape 108 beyond its equator. This providesa semiconstrained type of construction having a greater depth thanpresently available. Such a bearing can be used with the othercomponents of the present invention to provide the advantages, discussedabove, of (1) producing a wider range of motion, e.g., on the order of135°, and (2) providing a level of friction characteristic of a smallball for the majority of the motions of the patient's limb.

FIGS. 24-25 show another embodiment of the present invention wherein aplastic bearing 12 is fitted at its rim 118 with a metal reinforcingband 120 to produce a constrained joint able to withstand higherdislocation forces than, for example, the joints including plasticbearings shown in FIGS. 1-8 and 15-20. For this joint, the order ofassembly is first to combine bearing 12 with ring 74, then to force thebearing 12 over ball 10 and finally to assemble band 120 to the rim ofbearing 12. If the ball portion of the joint has already been implantedin the patient, this assembly order means that band 120 has to be placedover ball 10 before the bearing is mounted onto the ball.

The constructions of FIGS. 6-8, 9-10, 15-20, 21-23 and 24-25 illustratevarious configurations which can be interchangeably connected to body 64by means of bayonet lugs 70 and 78 on body 64 and ring 74, respectively.These configurations share the common characteristic that bearing 12 isfree to rotate about stub pins 34 in the final assembled joint. Theydiffer from one another in the degree of constraint imposed on the ballby the bearing and/or the types of materials used to construct thebearing. Because the bearings are free to rotate, these joints, whenapplied to hip reconstructions, also share the characteristic that thereis a preferred orientation of the joint with respect to the patient'sanatomy, namely, an orientation wherein most of the highly repetitiveload bearing motions of the joint occur along or close to the axis ofrotation of the bearing.

FIGS. 26-34 show further examples of joint components which can beinterchangeably connected to body 64 by means of bayonet lugs 70 and 78on body 64 and, in this case, on bearing 12, respectively. These jointconfigurations have the common characteristic that bearing 12 isstationary with respect to body 64. As with the joints of the priorexamples, these joints differ among themselves in the degree ofconstraint imposed on the ball by the bearing and/or the types ofmaterials used to construct the bearing.

More particularly, FIGS. 26-30 show a construction of the constrainedtype employing a plastic socket bearing to which has been added a metalreinforcing band 120 at rim 118 of the bearing so as to increase thebearing's ability to withstand dislocation forces. As shown in thefigures, the bearing includes a recess 126 at its pole which allows thebearing to be used with a body 64 which includes a polar pin 50. Ofcourse, if body 64 does not have a polar pin, the bearing need not haverecess 126.

Assembly of a completed joint using this bearing is accomplished asfollows. First, bearing 12 is forced over ball 10 and band 120 is forcedover the bearing's rim 118. If the ball portion of the joint has alreadybeen implanted in the patient, band 120 must be placed over ball 10before the bearing is mounted onto the ball. The sub-assembly of ball10, bearing 12 and reinforcing band 120 is then inserted into body 64 inany of the several angular positions the bayonet lug fittings willpermit. Polar pin 50 is received in recess 126 during this process.Since the bearing of FIGS. 26-30 is symmetric and thus does not define aparticular orientation with respect to the patient's anatomy, thespecific oritentation of the bearing with respect to body 64 isimmaterial. Once placed into body 64, a fraction of a turn in eitherdirection will engage the lugs 78 on the bearing under the lugs 70 ofbody 64. If desired, as described above, lugs 78 can be beveled ateither their right or left hand leading edges so that insertion byrotation in only one direction is facilitated, e.g., clockwise rotation.To aid in the rotation of bearing 12, the bearing can include slots 128for engagement with a spanner wrench or the like. The engagement ofbayonet lugs 78 and 70 is locked by one or more screws 83 which passthrough openings 100 and 98 in lugs 70 and 78, respectively, and thenthrough holes 102 in body 64 to engage the bone in which cup 64 has beenimplanted (see FIGS. 33 and 34).

FIG. 31 shows another plastic socket bearing of essentially the samedesign as the bearing of FIGS. 26-30 except that rather creating aconstrained joint, this bearing produces a semi-constrained joint. Thatis, the bearing of FIG. 31 does not encompass more than one half of ball10 and thus does not constrain the ball within the joint. Assembly of ajoint using this bearing simply involves inserting and locking thebearing in body 64 using bayonet lugs 78 and one or more bone screws,and then placing ball 10 into bearing 12 using the standard techniquesemployed in semi-constrained surgical reconstructions.

FIG. 32 shows another plastic socket for use in a semi-constrainedreconstruction, in this case with an added lip 130 to help restraindislocations of ball 10 from the bearing. The lip can be convenientlyformed by gradually sloping the front surface of the bearing as shown inFIG. 32. A suitable slope is on the order of 9°. Unlike the symmetricbearings of FIGS. 26-31, the bearing of FIG. 32 does define anorientation between itself and the patient's anatomy. Accordingly, wheninstalling this bearing, the surgeon will choose an orientation of thebearing with respect to body 64 which will place lip 130 in the mostadvantageous position to inhibit dislocation. Other than thisorientation step, the installation procedure for a joint using thisbearing is identical to the installation procedure described above forthe bearing of FIG. 31.

FIGS. 33-34 show a non-rotating, constrained joint employing a metalbearing. The bearing is composed of two half-bearings 132 which carrybayonet lugs 78. During assembly of the joint, half-bearings 132 areplaced about ball 10 and then held in place by plastic retaining ring134. The sub-assembly of ball 10 and bearing 12 is then inserted intobody 64 in basically the same manner as the bearing of FIGS. 26-30.Typically, body 64 is made of a titanium alloy, e.g., an alloycontaining 6% aluminum and 4% vanadium (see ASTM Spec. No. F136-79),while bearing halves 132, as well as ball 10, are made of acobalt-chromium alloy. As is known in the art, titanium alloys are notwear resistant when in moving contact with cobalt-chromium alloys andthus continual relative movement between a titanium part and acobalt-chromium part will eventually result in significant wearing ofthe titanium part.

To prevent such wearing due to relative movement, the joint of FIGS.33-34 includes wedge 136 which serves to force half-bearings 132 intotight engagement with body 64. Wedge 136 passes through slot 140 inbearing 12 and into recess 144 in retaining ring 134. The head portion148 of the wedge is struck with a hammer or the like after bearing 12has been positioned in body 64. The hammer blow forces the bearinghalves apart and locks them into position with respect to body 64.

To prevent wedge 136 from working loose over time, bone screw 83 can beused to engage the head of the wedge as shown in FIG. 34. So that thewedge can easily be removed, e.g., during replacement of the bearing bya different type of bearing, the underside of head 148 can be camberedso that the wedge can readily be pried away from the bearing. Becausethe wedging process causes the bearing halves 132 to move away from eachother by pivoting about their point of contact opposite the location ofthe wedge, the interior surfaces of the bearing halves can be contouredso that a spherical surface is formed only after the outward movement ofthe bearing halves. Alternatively, the interior surfaces can be madespherical since the slight amount of play between ball 10 and bearing 12caused by the wedging process can normally be tolerated in the completedjoint.

As can be seen most clearly in FIG. 33, edge 146 of bearing 12 is notsymmetric. More particularly, this edge allows ball 10 to move throughone angular range in the plane of FIG. 34 and a relatively largerangular range in the plane orthogonal to the plane of FIG. 34.Accordingly, when installing this bearing, the surgeon will choose anorientation of bearing 12 in body 64 which causes the planecorresponding to the relatively larger range of angular motion tocoincide as closely as possible with the plane in which the patient'slimb normally moves through its largest range of angular motion.

The nine joint configurations shown in FIGS. 6-8, 9-10, 15-20, 21-23,24-25, 26-30, 31, 32, and 33-34 are each interchangeable with eachother. This interchangeability gives the surgeon the flexibility ofbeing able to choose, in situ, any of these configurations or any otherconfiguration constructed to mate with body 64. In this way, a moreperfect match between the requirements of the patient and thecharacteristics of the artificial joint is achieved.

From the foregoing, it is evident that the present invention provides aconstrained ball and socket joint which has a range of motion greaterthan that generally available in artificial joints whether of theconstrained or semi-constrained type. Moreover, the present inventionprovides an artificial joint which can be oriented in the patient toprovide low friction movement of a ball of relatively small diameter formost of the patient's repetitive activities. The limiting factor inproviding the increased range of motion is the outside diameter of thebearing. Accordingly, within anatomical limits, it is advantageous forthe bearing outside diameter to be as large as possible.

The increased range of motion provided by the present invention allowsthe patient to move his limb further than heretofore possible inconstrained joints without the arm of the prosthesis impinging on theedge of the bearing. Accordingly, there is less likelihood ofdislocation when plastic bearings are used or disruption of the bondbetween the fixation element and the bone to which it is attached whenmetal bearings are used. Moreover, when the range of motion of thepresent joint is greater than the patient can take advantage of, thesurgeon is afforded greater latitude for variation in the orientation ofthe prosthetic components with respect to the patient's anatomy withoutthe hazard of impingement.

In addition to these orientability and range of motion aspects, thepresent invention, by providing interchangeable sockets, also gives thesurgeon the ability to select the most appropriate prosthesis for thepatient's specific requirements both during the initial operation and,if necessary, during re-operations. As demonstrated by the examplesdescribed above, the surgeon, in accordance with the present invention,has a diverse family of ball and socket joints to choose from extendingfrom semi-constrained, fixed bearings made of plastic throughconstrained, rotating bearings made of metal, as well as a variety ofconfigurations in between. This choice plainly permits a better matchbetween the requirements of the patient and the capabilities of theprosthesis.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. For example, ball 10, socketbearing 12 and retaining ring 74 can be provided to the surgeon as aunit, rather than being assembled at the surgical site. Also, metalsocket bearings can be used with one piece cups, such as cup 14 shown inFIG. 1, rather than with a retaining ring as shown in FIGS. 9 and 10.Furthermore, various connecting means other than bayonet spaces and lugscan be used to attach ring 74 or bearing 12 to body 64, such as, snaprings. In addition, a variety of socket bearings other than thosespecifically illustrated herein can be used to make up the family ofinterchangeable bearings supplied to the surgeon. It is thereforeunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A ball and socket joint for implantation in apatient's body comprising a ball portion and a socket portion,the ballportion including:a ball; and first fixation means for attachment to afirst bony structure, said fixation means being connected to said ball;and the socket portion including:a bearing for receiving the ball;second fixation means for attachment to a second bony structure, saidsecond fixation means having a cavity for receiving the bearing, saidcavity having an opening defining a plane through which the bearingenters the cavity; and means for securing the bearing to the secondfixation means in any one of a plurality of selectable orientationsafter the second fixation means has been attached to the second bonystructure, said plurality of selectable orientations being angularlydisplace from one another about an axis which is perpendicular to theplane defined by the opening of the cavity of the second fixation means;the bearing being non-symmetric with regard to rotation about said axisand said lack of symmetry making at least one of the selectable angularorientations of the bearing more preferred for physiological reasonsthan others of said angular orientations, said means for securingallowing said bearing to be secured to said second fixation means insuch a preferered orientation after said second fixation means has beenattached to the second bony structure.
 2. The ball and socket joint ofclaim 1 wherein the means for securing comprises bayonet spaces andlugs.
 3. The ball and socket joint of claim 1 wherein the means forsecuring includes two coaxial pin members and the bearing includes twocoaxial cylindrical surfaces which receive the pin members, the pinmembers and the cylindrical surfaces allowing the bearing to rotatewithin the cavity of the second fixation means about a single axis, saidsingle axis being orthogonal to the axis which defines the plurality ofselectable angular orientations, the orientation of the single axis withrespect to the anatomy of the patient's body making at least one of theselectable angular orientations of the bearing more preferred forphysiological reasons than others of said angular orientations.
 4. Theball and socket joint of claim 1 wherein the bearing includes a lip torestrain dislocations of the ball from the bearing, the orientation ofsaid lip with respect to the anatomy of the patient's body making atleast one of the selectable angular orientations of the bearing morepreferred for physiological reasons than others of said angularorientations.
 5. A prosthesis for implantation in a patient's bodycomprising:a bearing for receiving the ball portion of a ball and socketjoint; fixation means for attachment to a bony structure, said fixationmeans having a cavity for receiving the bearing, said cavity having anopening defining a plane through which the bearing enters the cavity;and means for securing the bearing to the fixation means in any one of aplurality of selectable orientations after the fixation means has beenattached to the bony structure, said plurality of selectableorientations being angularly displaced from one another about an axiswhich is perpendicular to the plane defined by the opening of the cavityof the fixation means; the bearing being non-symmetric with regard torotation about said axis and said lack of symmetry making at least oneof the selectable angular orientations of the bearing more preferred forphysiological reasons than others of said angular orientations, saidmeans for securing allowing said bearing to be secured to said fixationmeans in such a preferred orientation after said fixation means has beenattached to the bony structure.
 6. The prosthesis of claim 5 wherein themeans for securing comprises bayonet spaces and lugs.
 7. The prosthesisof claim 5 wherein the means for securing includes two coaxial pinmembers and the bearing includes two coaxial cylindrical surfaces whichreceive the pin members, the pin members and the cylindrical surfacesallowing the bearing to rotate within the cavity of the fixation meansabout a single axis, said single axis being orthogonal to the axis whichdefines the plurality of selectable angular orientations, theorientation of the single axis with respect to the anatomy of thepatient's body making at least one of the selectable angularorientations of the bearing more preferred for physiological reasonsthan others of said angular orientations.
 8. The prosthesis of claim 5wherein the bearing includes a lip to restrain dislocations of the ballfrom the bearing, the orientation of said lip with respect to theanatomy of the patient's body making at least one of the selectableangular orientations of the bearing more preferred for physiologicalreasons than others of said angular orientations.